On this page
Battery Storage: Your Ally in Poor Weather
Introduction: The Energy Buffer for Your Home
Solar systems, alongside wind turbines, are the symbol of sustainable energy generation. They are compact, quiet, powerful, low-emission and versatile. However, they have one major disadvantage: Weather dependency.
When clouds cover the sun, days are short, or snowstorms cover the modules, little to no usable solar electricity is generated. Thanks to battery technology, there is a solution: modern battery storage can bridge these periods.
The Battery in Daily Life: A Typical Day
The illustration below shows the interplay between time of day, solar electricity generation and energy consumption in a typical household.
Morning (6–9 am)
- Solar irradiance: Low (low sun angle)
- Electricity generation: Limited
- Consumption: Moderate (breakfast, hot water)
- Battery: Begins to discharge or grid draw
Late Morning to Midday (9 am – 2 pm)
- Solar irradiance: High to maximum
- Electricity generation: Maximum (sun at zenith)
- Consumption: Low (family away)
- Battery: Charges – stores surplus energy
Afternoon and Evening (3–10 pm)
- Solar irradiance: Decreasing to zero
- Electricity generation: Continuously declining
- Consumption: High (cooking, entertainment, heating)
- Battery: Discharges and supplies the house
Reality: Not Every Day is Ideal
The above example shows an ideal daily pattern. In practice:
- Everyone lives differently
- Weather is unpredictable
- Sometimes the battery doesn't fully charge
- Sometimes consumption doesn't allow optimal discharge
This is why intelligent charging and measurement electronics are employed. Smart software controls the power electronics and uses data from the electricity meter to deploy the battery as efficiently as possible.
Why a Storage at All?
Maximise Self-Consumption
Without storage: Surplus solar electricity flows to the grid (feed-in tariff ~8p/kWh) With storage: Own electricity is used in the evening (grid purchase ~30–35p/kWh)
Savings per kWh self-consumption: ~25p
Increase Self-Sufficiency
| System | Self-Sufficiency Rate |
|---|---|
| PV only, without storage | 25–35% |
| PV + storage | 60–80% |
| PV + large storage | up to 90% |
Grid Independence
A storage can serve as emergency power supply during power cuts (depending on the system).
Battery Sizing: How Large Must the Storage Be?
The correct storage size depends on several parameters:
Important Questions First
- How much electricity does the solar system generate on average?
- What is the maximum generation capacity (kWp)?
- What self-sufficiency rate do you want to achieve?
- How much electricity is consumed annually?
Rules of Thumb for Sizing
Based on peak capacity (kWp):
Per generated kWp, 0.9 to 1.6 kWh of storage capacity should be available.
| System Size | Recommended Storage |
|---|---|
| 5 kWp | 4.5 – 8 kWh |
| 8 kWp | 7.2 – 12.8 kWh |
| 10 kWp | 9 – 16 kWh |
Based on annual electricity consumption:
Capacity should be approximately 60% of daily electricity consumption.
| Annual Consumption | Daily Consumption | Recommended Storage |
|---|---|---|
| 3,000 kWh | 8.2 kWh | ~5 kWh |
| 5,000 kWh | 13.7 kWh | ~8 kWh |
| 7,000 kWh | 19.2 kWh | ~12 kWh |
Practical Tip
Oversizing rarely pays off:
- An oversized storage is never fully charged
- The additional costs are not recouped
- Better: Size slightly smaller and use the grid as backup
Understanding the C-Rate
The C-rate describes the ratio between charge/discharge power and storage capacity:
C-rate = Power (kW) / Capacity (kWh)
Example Calculation
A battery with:
- Discharge/charge power: 10 kW
- Capacity: 20 kWh
Has a C-rate of: 10 kW / 20 kWh = 0.5C
This means: The battery charges or discharges in 2 hours.
C-Rate Overview
| C-Rate | Charge/Discharge Time | Application |
|---|---|---|
| 0.25C | 4 hours | Slow charging, gentle |
| 0.5C | 2 hours | Standard for home storage |
| 1C | 1 hour | Fast charging |
| 2C | 30 minutes | High-performance storage |
Higher C-rates enable fast charging but stress the battery more and can shorten lifespan.
Battery Metrics at a Glance
Capacity (kWh)
The amount of energy the storage can absorb and deliver.
- Gross capacity: Total physical capacity
- Net capacity: Actually usable (usually 90–95%)
Charge and Discharge Power (kW)
How quickly the battery can absorb or deliver energy.
- Important for load peaks (e.g. switching on an electric cooker)
- Typical: 3–10 kW for home storage
Efficiency (%)
How much of the stored energy can actually be retrieved.
- Lithium-ion: 90–95%
- Losses occur through conversion and heat
Cycle Lifespan
How many charge/discharge cycles the battery can withstand.
- Typical: 5,000–10,000 cycles
- At one cycle per day: 13–27 years
Depth of Discharge (DoD)
How far the battery may be discharged.
- Lithium-ion: 80–100% DoD possible
- Higher DoD = more usable capacity, but more wear
Storage Technologies Compared
Lithium-Ion (Standard)
- Advantages: High energy density, long lifespan, high efficiency
- Disadvantages: Higher costs, temperature sensitivity
- Application: Standard for home storage
Lithium Iron Phosphate (LFP)
- Advantages: Very safe, long lifespan, robust
- Disadvantages: Slightly lower energy density
- Application: Increasingly for home storage
Lead-Acid
- Advantages: Cheaper, proven technology
- Disadvantages: Shorter lifespan, fewer cycles, heavier
- Application: Still in older systems, off-grid
Saltwater Batteries
- Advantages: Environmentally friendly, non-flammable
- Disadvantages: Lower energy density, heavy
- Application: Special applications
Conclusion
Key Point: A battery storage makes a solar system complete. It bridges the gap between generation (daytime) and consumption (evening), increases self-consumption and improves the economics of the system. When sizing: Don't choose too large. The rules of thumb (0.9–1.6 kWh per kWp or 60% of daily consumption) provide good guidance.
Continue reading: In the final article of this series Key Figures for Solar Systems: The Glossary, you will find all important metrics from kW to kWp, from efficiency to C-rate, clearly summarised.